Sheila, Nancy, Georgia

Last week (21/07/2022) Sheila and I looked into the effect of changing OMC POS Y offsets on DARM. First attachment shows the quiet spectra, optical gain (inverse), and SRCL-cleaned spectra. The optical gain obtained from this at 410.3 Hz (PCAL_Y line) is plotted as a function of the offset in the bottom panel on the second plot. The third attachment is the spectra at various offsets in the presence of excitation (1000 counts on OMC-ASC_POS_Y_EXC) - the top panel is DARM spectra, and bottom panel is the effect of excitation on OMC-ASC_QPD_A_PIT_OUT. I show this channel because this is the channel used by Georgia and Sheila in 49875 to calculate the coupling coefficient. The green curve is in the presence of excitation and does not change if the offset is changed. Using these DARM spectra and the QPD spectra, I take rms in the frequency range of 40-45 Hz (shown two vertical lines in the top panel in third attachment). These rms values are then used to calculate the coupling coefficient as follows: sqrt((DARM_exc**2 - DARM_quiet**2)/(QPD_exc**2-QPD_quiet**2)).

Overall, the behavior is similar to what was observed in 2019 by Georgia and Sheila: the offset at which minimum coupling is obtained is higher than the offset at which maximum optical gain is observed. There is a factor of ~1e-12 difference in the coupling coefficient between Georgia's plot and mine, and we are trying to figure out what is causing that.